This application claims priority of Korean Patent Application No. 0208507, filed May 7, 1997.
The present invention relates to an active noise control system, and particularly to a device for canceling periodic noises generated by electro-mechanical rotating mechanisms, such as washing machines, refrigerators, air conditioning systems and the like.
Most homes now have a variety of laborsaving and convenience devices, such as washing machines, spin-driers, refrigerators, air conditioning systems, swimming pool pumps and so forth. These devices ultimately depend on some type of electro-mechanical rotating device for functioning. Most of these devices are also in continuous use. A refrigerator for instance, runs continuously throughout the year, though the washing machine may only be used once or twice a week. During summertime, the drone of air conditioning compressors is a constant reminder of their existence. All of these devices have a common problem; they all make a lot of noise. Some noises have been shown to be detrimental to the health of some people. Low frequency noises in particular, cause headaches and in some cases will cause some form of nausea or upset stomach. Clearly, a device that can reduce the effects of these low frequency noises and vibrations will improve the general well being of the inhabitants of the home or workplace where these noises are present.
In addition to the above, one of the problems associated with any type of rotating machine such as those suggested above, is that the housings or enclosures surrounding these mechanisms are normally vented to allow free circulation of air for cooling purposes. These same vents that allow air to circulate also allow the noise to escape. Various degrees of passive noise reduction have been tried, with greater or lesser success, for many years. But adding soundproofing materials to a motor enclosure restricts cooling air flow. Therefore, a compromise has to be reached where the sound level is acceptable, and the cooling effect on the motor is reasonable for the duty cycle involved on the motor. Vibration-isolation motor mounts, and vibration absorbing foam goes some way toward correcting the noise pollution emitted by these mechanisms, but the plurality of ventilation openings cut in the enclosure of the machine negates the small effects each one provides. But the ventilation ports are not the only source of the noise. The enclosure of such equipment, especially a refrigerator or washing machine, acts as a sounding board for these low frequency noises, and even amplifies the noise by spreading out the sound source to a larger surface area.
There have been many attempts at reducing the noise output of machinery of all types for many years. Most of the methods tried have been passive approaches; e.g., sound absorbing foams, insulating blankets, baffles and so forth. The improvements in these acoustic insulators in recent years has been beneficial in a lot of cases but, more often than not, the higher frequencies are more readily absorbed by such measures, leaving lower frequencies relatively unchanged. This is partly due to the fact that the casing of the machine in question has some resonance at the lower frequencies, and tends to accentuate them, so they become more apparent.
With the advances in electronics in recent years, and with digital signal processing, in particular, several other inventors have shown various ways of creating an anti-noise signal, which, when applied in the right manner, can indeed reduce the sound level of the noise source. Most of these designs have been centered around the digital signal processing techniques so popular now.
The prior art includes numerous patents for noise reduction devices. For instance U.S. Pat. No. 4,953,217, issued to Twiney et al. on Aug. 28, 1990, teaches a noise reduction system for use in ear protection headsets. U.S. Pat. No. 5,125,241, issued to Nakanishi et al. on Jun. 30, 1992, describes a noise attenuation device for a refrigerator. U.S. Pat. No. 5,129,003, issued to Saruta on Jul. 7, 1992, teaches an active noise control apparatus for domestic appliances which generates a sound wave having an opposite phase and amplitude to the noise generated by the appliance. U.S. Pat. No. 5,140,640, issued to Graupe et al. on Aug. 18, 1992, teaches yet another noise cancellation system. U.S. Pat. No. 5,267,320, issued to Fukumizu on Nov. 30, 1993, teaches a noise control device in a movable system. U.S. Pat. No. 5,365,594, issued to Ross et al. on Nov. 15, 1994, teaches an active sound control device primarily for vibration control. U.S. Pat. No. 5,381,485, issued to Elliott on Jan. 10, 1995, describes another active sound control device. U.S. Pat. No. 5,485,523, issued to Tamamura et al. on Jan. 16, 1996, teaches a noise reduction device for use in an automobile. U.S. Pat. No. 5,488,667, issued to Tamamura et al. on Jan. 30, 1996, discloses another noise reduction system for use in a automotive vehicle. U.S. Pat. No. 5,491,747, issued to Bartlett et al. on Feb. 13, 1996, describes a noise reducing telephone handset using passive means to cancel the noise. U.S. Pat. No. 5,493,616, issued to Iidaka et al. on Feb. 20, 1996 describes another device for reducing the noise generated by an automotive vehicle. U.S. Pat. No. 5,499,301, issued to Sudo et al. on Mar. 12, 1996 describes an active noise cancellation apparatus for controlling noise generated by a compressor. U.S. Pat. No. 5,508,477, issued to Kato et al. on Apr. 16, 1996, covers an apparatus for reducing noise in an office utilizing a Helmholtz resonator. U.S. Pat. No. 5,539,831, issued to Harley on Jul. 23, 1996, describes an active noise control processor for use in a stethoscope. U.S. Pat. No. 5,546,467, issued to Denenberg on Aug. 13, 1996, teaches another noise attenuation device for use with domestic appliances. U.S. Pat. No. 5,559,893, issued to Krokstad et al. on Sep. 24, 1996, describes another noise reduction device which uses microphones and loud speakers. U.S. Pat. No. 5,581,619, issued to Shibata et al. on Dec. 3, 1996, discloses another noise reduction system for use within a vehicle. U.S. Pat. No. 5,600,729, issued to Darlington et al. on Feb. 4, 1997, discloses an active noise control system for use within an air cover. U.S. Pat. No. 5,602,927, issued to Tamamura et al. on Feb. 11, 1997, covers another noise reduction system for use within an automotive vehicle. Finally, U.S. Pat. No. 5,619,581, issued to Ferguson et al. on Apr. 8, 1997, describes another active noise and vibration control system which uses digital signal processors.
Unlike the above patented noise reduction/cancellation devices the present invention employs a simpler method of obtaining the same results, without the need to actually generate an anti-phase signal required for noise cancellation. The invention is manually adjustable so it can be used in a wide variety of noise frequency situations. Being adjustable, it can then be adjusted to give the most effective noise cancellation possible for the particular location and application. Furthermore, the device may be installed on new equipment prior to being sold, or may be bought and installed as a separate aftermarket add-on device for older equipment.
The primary object of the present invention is to provide domestic appliances and other equipment with a noise attenuating function wherein the audible noise generated by the electro-mechanical device driving the equipment is actively attenuated.
Another object of the present invention is to provide such a noise attenuating apparatus that is controllable by the user of the equipment, or by the agent installing the device.
An even further object of this invention is to provide such an apparatus which can be either installed in the electro-mechanical device when made or during use later.
The present invention accomplishes the above and other objects by providing an apparatus that cancels ambient noise by having an input sensor means, an input amplifier, means for analyzing and dividing the signal, means for introducing a half cycle phase delay, means for recombining the signals into one signal with an output amplifier for passing the signal to an output loudspeaker to effectively cancel the ambient noise. The sensor means picks up ambient noise and converts it into an electrical input signal containing amplitude and temporal information corresponding to the frequency wave of the ambient noise. The input signal is then fed through an input amplifier to a series of digital filters which divide the signal into a fundamental signal and a series of separate harmonic signals of different frequency ranges. Then each of the harmonic signals is fed through delay lines which introduce a half cycle phase delay to the signals. Then the signals are combined into an output signal which is then amplified and passed to a transducer to yield an output noise having a frequency wave which is shifted one half cycle from the frequency of the ambient noise such that the ambient noise is canceled. The means for introducing the phase delay may be adjustable by a potentiometer by which one can manually alter the phase shift before the input and output signal to achieve the ultimate sound reduction. The system also contains a feed back loop to prevent the apparatus from canceling out the output noise of the apparatus itself. The method of canceling noise by introducing a half cycle phase shift is also covered.
The above and other objects, features and advantages of the present invention should become even more readily apparent to those skilled in the art upon a reading of the following detailed description in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention.